1. Field of the Invention
The present invention relates to a display apparatus, a pixel layout method for a display apparatus, and an electronic device. More particularly, the present invention relates to: a planar (i.e., flat panel) display apparatus wherein pixels having electro-optical elements are arranged in a two-dimensional array, a pixel layout method for such a display apparatus, and an electronic device including such a display apparatus.
2. Description of the Related Art
Recently, the field of display apparatus for displaying images has seen rapid uptake of planar display apparatus, wherein pixels having light-emitting elements (hereinafter also referred to as to pixel circuits) are arranged in a two-dimensional array. In one such type of planar display apparatus, current-driven electro-optical elements are used as the light-emitting elements of the pixels. In other words, the luminous intensity of these elements changes according to the value of current flowing into the device. One example of such current-driven electro-optical elements is organic EL (electro luminescence) elements, which utilize the phenomenon of light emission as a result of applying an electric field to an organic thin film.
Organic EL display apparatus using organic EL elements as light-emitting elements have the following advantages. Since organic EL elements can be driven with applied voltages of 10 V or less, power consumption is low. Since organic EL elements are self-luminous, image visibility is higher compared to that of a liquid crystal display apparatus that displays images by controlling, on a per-pixel basis, the luminous intensity of light from a light source passing through liquid crystals. Moreover, since the backlight or similar light source can be omitted, it is easy to make organic EL display apparatus lighter and thinner. Furthermore, the response time of organic EL elements is very fast, on the order of several microseconds. For this reason, ghosting does not occur when displaying video.
Similarly to liquid crystal display apparatus, organic EL display apparatus may adopt a simple (passive) matrix system or an active matrix system as the driving system. However, while passive-matrix display apparatus are simple in construction, the light-emitting period of the electro-optical elements decreases with increases in the number of scan lines (i.e., the number of pixels), making it difficult to realize display apparatus that are both large and high-definition.
Consequently, development of active-matrix display apparatus is currently popular. In an active-matrix display apparatus, current flowing into each electro-optical element is controlled by an active element (such as an insulated-gate field-effect transistor, for example) provided within the same pixel as the given electro-optical element. Typically, a thin film transistor (TFT) is used as the insulated-gate field-effect transistor. In active-matrix display apparatus, the electro-optical elements maintain light emission for the duration of a single frame. For this reason, it is easy to realize large, high-definition displays.
In an active-matrix organic EL display apparatus, each pixel (i.e., pixel circuit) is configured to at least include a drive transistor, a write transistor, and a hold capacitor as the driving circuit of the organic EL element (see, for example, Japanese Unexamined Patent Application Publication No. 2005-345722). The drive transistor current-drives the organic EL element. The write transistor samples a picture signal and writes within the pixel. The hold capacitor holds the picture signal written by the write transistor.
Meanwhile, display apparatus are undergoing increases in definition and decreases in power consumption in recent years. As the definition of display apparatus increases, the size of each organic EL element decreases, which also results in smaller capacitance values for the parasitic capacitance of each organic EL element. In addition, as the power consumption of a display apparatus is decreased, the amplitude of the picture signals written to the pixels is designed to be lower.
During a picture signal write operation performed by the write transistor, when the gate potential Vg of the drive transistor rises due to writing the picture signal, the source voltage Vs of the drive transistor also rises, due to coupling between the hold capacitor and the parasitic capacitance of the organic EL element. The amount of increase ΔVs in the source voltage at this point can be expressed asΔVs=ΔVg×{Ccs/(Ccs+Cel)}  (1)where ΔVg is the amount of increase in the gate potential, Ccs is the capacitance value of the hold capacitor, and Cel is the capacitance value of the parasitic capacitance of the organic EL element.
Pixel miniaturization as a result of increasing the definition of a display apparatus causes the capacitance value Cel of the parasitic capacitance of the organic EL element to decrease. As the above Eq. 1 demonstrates, if the capacitance value Cel decreases, then the amount of increase ΔVs in the source voltage of the drive transistor also increases. Consequently, the driving voltage of the drive transistor (i.e., the gate-to-source voltage Vgs) decreases. As a result, a luminous intensity corresponding to the amplitude of the input picture signal is not obtained. Although the decrease in luminous intensity can be counteracted by increasing the amplitude of the picture signal, doing so impedes the lowering of power consumption in the display apparatus.
The foregoing thus describes an existing problem, taking by way of example the case where organic EL elements are used as the electro-optical elements. However, the above problem is not limited to organic EL elements, and can be said to apply generally to electro-optical elements having parasitic capacitances.
In order to compensate for insufficient parasitism in an electro-optical element, configurations have been adopted wherein an auxiliary capacitor is added between the anode of the electro-optical element (i.e., the source electrode of the drive transistor) and a fixed potential node (see, for example, Japanese Unexamined Patent Application Publication No. 2008-051990). This auxiliary capacitor compensates for insufficient capacitance in the electro-optical element, even for small capacitance values for the parasitic capacitance of the electro-optical element. In so doing, the auxiliary capacitor acts to suppress rises in the source voltage Vs of the drive transistor when writing signals. As a result of the action of the auxiliary capacitor, the driving voltage of the drive transistor can be secured without increasing the amplitude of the picture signal.